In a heater head of the kind defined it has been common practice to form the said ring of a diameter corresponding to or greater than the horizontal extent of the combustion chamber. In hot gas engines the non-variable spaces containing working gas i.e. the spaces not limited by the pistons and the cylinder walls, should be kept as small as possible. Also the surfaces exposed to heat by radiation from the combustion chamber should be as large as possible, whereas the resistance against the gas flow inside the heat exchanger tubes should be as small as possible. Modifications of design to achieve advantages of one kind nearly always cause disadvantages in other respects, and therefore it is an object of the present invention to provide a heater head of the kind defined which can be constructed to give good efficiency without involving severe disadvantages, for example in respect of cost.